In August, as amateurs and professionals alike turned their telescopes on the nearest Type Ia supernova discovered in decades, University of California, Berkeley, research astronomer Weidong Li focused instead on what could not be seen.

The supernova SN 2011fe discovered on August 24 was located in the Pinwheel Galaxy in the constellation of the Big Dipper (Ursa Major). (Image by B. J. Fulton, Las Cumbres Observatory Global Telescope Network.)

Li pulled up images of the northern sky taken over the past nine years by the Hubble Space Telescope in search of the star and its binary companion as they looked before the supernova explosion. But his initial disappointment when he saw no star at all quickly turned to excitement.

Because the Hubble telescope is so sensitive, the fact that it did not detect any stars in the area in the Big Dipper told Li and his colleagues that neither the pre-supernova “progentitor” star – presumably a white dwarf – nor its companion were bright. In fact, Li’s analysis of the archival high-resolution Hubble telescope data suggests that the companion to the Aug. 24 supernova could not have been a bloated red giant or a bright helium star, which would have been visible.

Instead Li and his colleagues conclude in a paper published in the Dec. 15 issue of the journal Nature that the companionwas most likely a normal star like the sun, a somewhat evolved star called a subgiant, or perhaps a white dwarf.

Li died unexpectedly on Monday, Dec. 12, saddening his collaborators and many colleagues in the UC Berkeley astronomy department.

Li’s discovery about the supernova’s origins is helping astronomers understand more about the explosive mechanisms of Type Ia supernovae, which are the foundation of discoveries for which three scientists – including UC Berkeley physics professor Saul Perlmutter – last weekend were awarded the 2011 Nobel Prize in Physics.

“Our paper is the first ever to exclude directly some of the major candidates for Type Ia supernovae,” said coauthor Joshua Bloom, UC Berkeley associate professor of astronomy.

The team’s finding fits with new theoretical modeling of the exploding white dwarf by the supernova’s discoverers: Peter Nugent, head of the Computational Cosmology Center at Lawrence Berkeley National Laboratory, and his colleagues, including physicists from UC Berkeley. They also published their conclusions in the Dec. 15 issue of Nature.

Type Ia supernovae indicate accelerating expansion of universe

Type Ia supernovae are bright and visible across huge cosmic distances, which allowed Perlmutter, his Nobel laureate colleagues and their teams to employ them as “standard candles” to measure the universe. In 1998, these studies revealed that the expansion of the universe is accelerating, and such acceleration is now widely believed to require the presence of a mysterious “dark energy.”

“The discovery of the accelerating expansion of the Universe has revolutionized physics, and the repulsive dark energy may provide key clues to the long-sought quantum theory of gravity,” said coauthor Alex Filippenko, who was a member of both of the teams that made the Nobel Prize-winning discovery and a UC Berkeley professor of astronomy. “But the actual origins of Type Ia supernovae have remained mysterious, and various aspects of the explosion are not well understood.”

Astronomers believe that the star that explodes to produce a Type Ia supernova is a white dwarf composed primarily of carbon and oxygen, but the white dwarf presumably explodes because it pulls matter from a binary companion that astronomers to date have been unable to identify.

An opportunity to study such explosions at close hand came on Aug. 24, when Nugent, while looking at data from the Palomar Transient Factory (PTF), which searches for short-lived events in space, spotted a remarkable object in the nearby Pinwheel Galaxy (Messier 101) some 21 million light years away. Subsequently named SN 2011fe, it garnered worldwide attention as the nearest such supernova in the past 25 years. Although its brightness was 40 times too faint for the eye to see, for a month the supernova could be easily viewed through binoculars, a rare occurrence among amateur astronomers and the general public.

“These are the sorts of important events that the Palomar Transient Factory was designed to uncover,” said PTF principal investigator Shrinivas Kulkarni, a professor at the California Institute of Technology.

Li immediately thought of looking for the original star and its companion in past Hubble data, seeing it, he said, as “a golden opportunity to investigate the properties of the progenitor system through direct imaging.”

Within two days, Li and about 30 collaborators from around the world were able to obtain images of the supernova from the 10-meter Keck Telescope in Hawaii and its adaptive optics system. Using these high-resolution images, they pinpointed the exact location of the supernova, but when they saw nothing there, they used the red, green and blue wavelength observations from the telescope to set a stringent limit on how bright the progenitor and its companion could be.

The authors then were able to exclude the presence of a red giant and most types of helium stars, leaving a faint white dwarf or a subgiant star as likely companions.

Next steps include using more sensitive telescopes, such as the planned James Webb Space Telescope, to detect the faint surviving companion star.

“That will give us another opportunity to reveal more secrets about this supernova and ultimately help us understand the explosion physics of Type Ia supernovae, and possibly refine them as an even better cosmological distance ladder,” Li said.